no_clobber_xff option
Merge commit 'c7f57eda1cad71f49f7344a263dbf8a251f5bcde' into feature/remote-addr

Pull request #181

In handler mode, set special header REMOTE_ADDR containing the IP
address of the requesting client. Setting no_clobber_xff prevents
X-FORWARDED-FOR from being touched in handler mode.

Proxy mode remains unchanged. REMOTE_ADDR is not set in that context,
and X-FORWARDED-FOR is always set/replaced (no_clobber_xff has no
effect on proxy mode).
Merge branch 'feature/remote-addr' into develop

Closes #181